Enemy randomly spawn in an area, find a road or track and place an IED or varying power down. It then looks for a nearby buildings with LOS to hide in, if no buildings then a bush, no bush then random place with LOS.

Bomb does not detonate unless there are more than 1 playable units in the trigger zone.

You want to keep it to a base level so that the mission maker can decide how it is handled rather than giving him one option.The idea of setting it up like this allows modifiable scenarios, either it can be via the one guy or many, whether it is followed by an attack or just to disrupt. Or if it is just the remains from a previous mortar round that hasn't exploded.